The present technology relates to a data storage medium having an optical information carrier which comprises a polymer carrier set up to store information.
DE 298 16 802 describes a data storage medium having an optical information carrier which comprises a polymer film. As material for the polymer film, polymethyl methacrylate and also a polymer film sold by Beiersdorf AG under the designation “Tesafilm kristallklar”, which comprises biaxially oriented polypropylene, are specified. In the data storage medium, the polymer film is wound spirally in a plurality of plies onto a winding core, with an adhesion layer between each pair of adjacent plies. Information may be written to the data storage medium by locally heating the polymer film using a write beam of a data drive, as a result of which the refractive index and thus the reflecting power (reflectivity) at the interface of the polymer film are locally changed. This can be detected using a read beam in the data drive. By focussing the write beam or read beam, the information may be written to or read from, respectively, a specific preselected ply of the information carrier. The winding core may be optically transparent and may in its center have a recess which serves to accommodate the read/write device of a data drive. The read/write device is moved relative to the data storage medium, whereas the data storage medium is stationary, so there is no need for the data storage medium to be balanced to take account of rapid rotational movement.
With the existing data storage medium it is already possible to achieve very high storage densities. A further increase in the storage density is, however, desirable.
An exemplary embodiment of the invention provides a data storage medium having an optical information carrier which comprises a polymer carrier set up to store information, wherein the fundamental possibility exists of an increase in the storage density.
The data storage medium of an exemplary embodiment of the invention has an optical information carrier which comprises a polymer carrier set up to store information. The optical information carrier further comprises a layer which comprises a dye and can be optically changed locally in order to store information. In principle, the data storage medium may already have been written with information by the manufacturer; this applies both to the polymer carrier and to the dye layer. It is, however, also conceivable to configure the optical information carrier in such a way that a user is able to input information into the data storage medium, utilizing the polymer carrier and/or the dye layer.
Because the data storage medium comprises not only the polymer carrier set up to store information but also an additional layer which may be used to store information, in principle a very high storage density may be achieved. Embodiments particularly advantageous for this purpose are elucidated later on.
Preferably, the dye can be at least partly bleached out by means of a write beam. In this case, data (information units) can be written into the dye layer by locally bleaching out the dye in accordance with the information that is to be input, using intensive laser pulses, for example. The area for one stored information unit typically has a diameter or a side length of approximately 1 μm. As a result of the partial or complete bleaching-out of the dye, the optical properties at the affected site change distinctly, and this may be detected by a suitable read beam. If, for example, the frequency range of the read beam is attuned to the absorption range of the dye, the absorption of the read beam at a bleached-out site is less than in its vicinity, where there is dye. If, after penetrating the dye layer, the read beam is reflected at an interface of the information carrier, therefore, a distinct read signal is produced by the bleached-out sites. Additionally, changes in the absorption activity owing to the wavelength dependency of the refractive index, which is particularly strong in the region of an absorption band, result in changes in the refractive index. This gives rise locally to changes in reflectivity, and the phase position of a read beam is also influenced. All of this may be utilized in order to obtain a read signal.
The dye may be, for example, a cyanine, a phthalocyanine or a mixture of such substances.
Cyanines and phthalocyanines are currently used in writable CDs. Further examples of the dye are eosin, and eosin B. Besides the dye, the dye layer may also comprise other substances; for example, a binder—in this context, one advantageous embodiment is elucidated later on below.
In one preferred embodiment of the invention, the refractive index of the polymer carrier can be changed locally by heating. A polymer carrier having this property may be designed, for example, in the form of a stretched polymer film, which, for example, is pretensioned in two directions perpendicular to one another within its plane during production. Suitable material for the polymer film includes, for example, polymethyl methacrylate (PMMA) or, in particular, biaxially oriented polypropylene (BOPP), although other materials are also possible. Using a write beam, information units can be written to an information carrier comprising a polymer carrier of this kind. In the case of a stretched polymer film, a high energy density is stored in the film material. By depositing a relatively small amount of energy per unit area, using a write beam, it is then possible to obtain a sharp change in material (for example, a densification of material) by reformation, resulting in a local change in the refractive index and in a change in the optical path length in the material. In this way it is possible, for example, to achieve a change in the refractive index in the order of magnitude of 0.2 in the area locally heated by the write beam, and to do so over an area for a store information unit with a diameter or side length of approximately 1 μm. This leads to a change in the local reflectivity, which is readily detectable by means of a read beam. The details of the local optical change in the polymer carrier on storage of information therefore differ from those associated with the storage of information in the dye layer.
The polymer carrier may be assigned an absorber which is set up to absorb, at least partly, a write beam and to emit the generated heat, at least partly, locally to the polymer carrier. The absorber comprises, for example, dye molecules, which are present in the polymer carrier or in a layer adjacent to the polymer carrier, for example, an adhesion layer (see below) or else the above-elucidated dye layer, and permits a local heating of the polymer carrier, sufficient to change the refractive index, for a relatively low write beam intensity.
A further advantage of the invention is manifested if the information carrier is set up so that the frequency range of a read beam for reading information from the polymer carrier is different from the frequency range of a read beam for reading information from the dye layer. With this embodiment it is in fact possible for the dye layer to have stored in it, or to have had stored in it, data which can be detected only using a read beam whose frequency range has been attuned, for example, to the absorption in the dye, with the dye being virtually transparent to other frequency ranges. For instance, it is possible to use such a data storage medium in a drive attuned to it which has a read device and, optionally, a write device, the read device operating with a read beam whose frequency range is set up only to read information from the polymer carrier but not to read information from the dye layer. In the dye layer, therefore, it is possible to accommodate hidden data which cannot be read with the elucidated drive, something which offers advantages in certain application environments. If, on the other hand, the data storage medium is used in a drive which has a read device and, optionally, a write device, the read device operating with read beams whose frequency ranges have been set up to read information from the polymer carrier and to read information from the dye layer, this drive can be used to recognize all of the stored data, and it is possible to utilize the advantage that the data storage medium of the invention offers a particularly high storage density.
In one preferred embodiment of the invention, the information carrier comprises two or more polymer carrier plies (preferably polymer film plies) through which data or information units may be read from a preselected polymer carrier ply and, optionally, written to a preselected polymer ply. Preferably, there is an adhesion layer between each pair of adjacent polymer carrier plies, in order to fix the polymer carrier plies to one another. A suitable adhesion agent is, for example, an acrylate adhesive which is free from gas bubbles and is crosslinked, for example, chemically or by irradiation with UV or electron beams. If the refractive index of the adhesion layer differs only slightly from the refractive index of the polymer carrier, disruptive reflections of a read beam or write beam at a boundary layer between a polymer carrier ply and an adjacent adhesion layer are minimized. It is particularly advantageous if the difference in refractive indices is less than 0.005. An existing difference in refractive indices may, however, be utilized for the purpose of formatting the data storage medium. In one preferred embodiment, at least one adhesion layer is set up as a dye layer, which can be optically changed locally for the purpose of storing information. It is also possible for two or more, or all of the, adhesion layers between polymer carrier plies to be provided as layers of this kind for storing information. This construction of the data storage medium is particularly compact, since the adhesion layers are utilized not only for bonding the polymer carrier plies but also, at the same time, for information storage. In addition, the adhesion layers may comprise the abovementioned absorber which is set up so as to absorb heat from a write beam and emit it to the polymer carrier.
As the polymer carrier, it is possible to use plate material. As already mentioned, the polymer carrier may alternatively comprise a polymer film, made of biaxially oriented polypropylene (BOPP), for example.
In this case, in one preferred embodiment the information carrier is wound in a spiral fashion, onto an optically transparent winding core, for example, which has a recess in its central region. In this case there is preferably an adhesion layer between each pair of adjacent polymer film plies or winds, said adhesion layer containing dye for the purpose of storing information. In other words, therefore, the information carrier has a coherent adhesion layer which is wound in a spiral fashion in exactly the same way as the polymer film. For example, from 10 to 30 polymer film plies may be wound, or else a greater or lesser number. With a polymer film thickness of between 10 μm and 100 μm, preferably below 50 μm or around 35 μm, the information can be separated from one another with good resolution on different polymer film plies or on different dye plies of the adhesion layer by means of read/write devices that are known, for example, from DVD technology. The adhesion layer may, for example, have a thickness in the range between 1 μm and 40 μm, preferably below 25 μm.
If the data storage medium with spiral-wound information carrier has an optically transparent winding core which has a recess in its central area, it is possible to arrange in said recess a read device and, optionally, a write device of a drive that is attuned to the data storage medium and to move said device(s) relative to the data storage medium, while said data storage medium is stationary, in order to read or write information. A stationary data storage medium has the advantage that it does not have to be balanced in order to allow high rotational speeds, which has favorable consequences for the production costs.